[論文レビュー] Second harmonic study of thermally oxidized mono- and few-layer 2H-MoS2

Katharina Burgholzer, Henry Volker Hübschmann+6 more|arXiv (Cornell University)|Mar 9, 2026

2D Materials and Applications被引用数 0

ひとこと要約

The paper uses non-resonant second harmonic microscopy to monitor progressive thermal oxidation of MoS2 flakes (1–7 layers) up to 6 hours, revealing layer-dependent and oxidation-time-dependent changes in SHG, supported by DFT band-structure calculations.

ABSTRACT

A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.

研究の動機と目的

• Investigate how thermal oxidation alters the nonlinear optical response (SHG) of 2H-MoS2 flakes from 1L to 7L.
• Determine layer-dependent oxidation depth and symmetry changes induced by oxidation.
• Quantify how SHG changes with oxidation time under controlled O2 exposure and temperature.
• Correlate experimental SHG changes with band-structure modifications from first-principles calculations.

提案手法

• Perform SHG microscopy with controlled power and polarization to probe crystal symmetry and nonlinear response.
• Compare untreated and oxidized MoS2 flakes (0–6 h oxidation at 300°C) across 1L–7L, recording SH intensity and polarization dependence.
• Apply polarization-resolved measurements to identify six-fold symmetry and its changes due to oxidation.
• Use DFT (PBE, GGA, Grimme D3, SOC neglected) to model oxidized top S layer and analyze band-structure modifications.
• Correlate SHG fingerprints with oxidation depth and layer-number dependent electronic structure changes.

Figure 1: Crystal structure of 2H- $\text{MoS}{\vphantom{\text{X}}}_{\smash[t]{\text{2}}}$ (top) top view with indications of the armchair and zigzag directions and corresponding orientation of the nonlinear tensor elements $d_{yyy}$ and $d_{yxx}$ , (bottom) side view 2H stacking.

実験結果

リサーチクエスチョン

• RQ1 How does thermal oxidation affect the SHG response of 1L–7L MoS2?
• RQ2 Is the oxidation depth confined to the top sulfur layer, and how does it depend on layer thickness?
• RQ3 How do oxidation time and layer parity (odd vs even) influence SHG magnitude and polarization behavior?
• RQ4 What band-structure changes accompany oxidation, and how do they explain SHG trends?
• RQ5 Can SHG microscopy serve as a non-invasive monitor of oxidation progress in MoS2-based devices?

主な発見

• Oxidation causes significant changes in SHG for all layer thicknesses studied (1L–7L).
• Odd-layer MoS2 shows a reduction of SHG with oxidation, while even-layer MoS2, previously SHG-silent due to inversion symmetry, gains SHG signal from oxidation.
• SHG decreases progressively with oxidation time for odd layers and increases for even layers, indicating a layer-dependent oxidation depth limited to the top layer.
• Polarization-resolved SHG shows a six-fold symmetry both before and after oxidation, with the intensity range narrowing after oxidation, implying preferred oxidation directions while retaining symmetry features.
• DFT band-structure calculations show pronounced oxidation-induced band-structure changes, especially in 1L MoS2, including a direct-to-indirect gap transition and reduced SHG likelihood at the excitation energy used (783 nm).
• Overall, SHG microscopy provides non-resonant, non-invasive fingerprints of structural modification due to thermal oxidation in MoS2 across multiple thicknesses.

Figure 2: Optical images of $\text{MoS}{\vphantom{\text{X}}}_{\smash[t]{\text{2}}}$ flakes on $\text{SiO}{\vphantom{\text{X}}}_{\smash[t]{\text{2}}}$ /Si-substrate before (left) and after (right) 6h-oxidation at $300\text{\,}\mathrm{\SIUnitSymbolCelsius}$ . Here, the regions for the 1L and 3L system